[riscv-port] RFR: 8282415: riscv: Rename StubRoutines::riscv64 to StubRoutines::riscv

[Feilong Jiang]

Rename `SubRoutine::riscv64` to `SubRoutine::riscv`, just like the PPC64 port.

Tier1 tests on QEMU are passed without new faliures.

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Commit messages:
 - 8282415: riscv: Rename StubRoutines::riscv64 to StubRoutines::riscv

Changes: https://git.openjdk.java.net/riscv-port/pull/60/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=60&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282415
  Stats: 47 lines in 6 files changed: 1 ins; 0 del; 46 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/60.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/60/head:pull/60

PR: https://git.openjdk.java.net/riscv-port/pull/60